Aminotransferases conserved structure

Mechanisms. Studies of model reactions473-476 and of electronic, Raman,456 477 478 ESR,479/480 and NMR spectra and kinetics481 have contributed to an understanding of these enzymes.459 461 464 482 483 For these copper amine oxidases the experimental evidence suggests an aminotransferase mechanism.450 453 474 4743 d Tire structure of the E.coli oxidase shows that a single copper ion is bound by three histidine imidazoles and is located adjacent to the TPQ (Eq. 15-53). Asp 383 is a conserved residue that may be the catalytic base in Eq. 15-53.474b A similar mechanism can be invoked for LTQ and TTQ. 

Aspartate aminotransferase is the prototype of a large family of PLP-dependent enzymes. Comparisons of amino acid sequences as well as several three-dimensional structures reveal that almost all transaminases having roles in amino acid biosynthesis are related to aspartate aminotransferase by divergent evolution. An examination of the aligned amino acid sequences reveals that two residues are completely conserved. These residues are the lysine residue that forms the Schiff base with the pyridoxal phosphate cofactor (lysine 258 in aspartate aminotransferase) and an arginine residue that interacts with the a-carboxylate group of the ketoacid (see Figure 23.11). 

Fig. 5. Eight highly conserved regions of the primary structure of ACC synthase. Identical amino residue relative to the sequence of CM-ACSl (from Cucurbita maxima) is shown by dot. Eleven resuidues shown to be invariant among ACC synthase and aminotransferase are marked with X. The lysine residue (Box 6) which interacts with pyridoxalphosphate and with AdoMet is marked with. However, tyrosine residue in Box 5 are replaced by phenylalanine in AVO-ACSl (from avocado) and DC-ACSl (from carnation). The numbers between the adjacent boxes show number of residues present. AVO, avocado (Persea americana Mill) MELW, melon (Cucumis melo L.) GAC, geranium (Pelargonium x hortorum). For other abbreviation of source plants, see Table 1.

Figure 37 The SMAT family of enzymes schematic representation of primary structural similarities, (A) The aminotransferase subfamily contains a conserved lysine (K) residue. (B) The dehy-drase subfamily (DH-E,) contains a histidine (H) residue in the same position and several conserved cysteine (C) residues, mostly concentrated in an insert. The N-tcrminal region of similarity (stippled) arid two commonly conserved motifs in both sequences are indicated.

---